Fri October 4 lecture notes

(1) Robots

(2) Java Recap:

 * Classes:     define behavior of type of "object", has data and methods
 * Methods:     defining them, calling them, passing arguments to them
 * Variables:   different types (int, double, String, ...) of data
 * Arguments:   are a temporary name in definition, passed when called
                like in algebra formulas, i.e.      
                      f(x)=x+1    // here "x" is a temporary name
                      f(2)        // which is given a value
                In Java, this is
                      --- in file Function.java ---
                      Class Function {
                        public double f(double x){ return x+1; }
                      }
                      --- in TestFunction.java ---
                      class TestFunction {
                        public static void main( String[] args ) {
                          Function thisFunc = new Function();
                          double answer = thisFunc.f(2);
                          System.out.println( " f(2) is " + answer );
                        }
                      }
                Compile with " javac Function.java; javac TestFunction.java "
                Run with "java TestFunction"
                 
 * Invoking methods : object.method(arguments)

 An example:

 ---- file GuyWhoLikesToTalk.java -----

   // An object which can print out a string several times.
   public class GuyWhoLikesToTalk {

      // constructor for PrintStuff object 
      GuyWhoLikesToTalk(){
      }

      // A method of this object.
      // In other words, a recipe for something that a PrintStuff can do.
      public void sayThreeTimes(String whatToSay){
          System.out.println( " 1st time: " + whatToSay );
          System.out.println( " 2nd time: " + whatToSay );
          System.out.println( " 3rd time: " + whatToSay );
      }

      // Another method.
      public void tellAJoke(){
          System.out.println( " Well, this guy walks into a bar.  Thud. ");
      }

   }

 ---- file TestPrintStuff.java ---------

   // A class which calls the PrintStuff object
   public class TestPrintStuff {
   
      // main() method, i.e. what gets called when the program runs.
      public static void main( String[] args ){

         PrintStuff george = new PrintStuff();
	 String     hello  = "Hello out there in the real world."

	 System.out.println(" george will now speak.  Say hello, george.");   
         george.sayThreeTimes( hello );

	 System.out.println(" george will now tell a joke.");   	 
	 george.tellAJoke();
 
      }
   }

(3) New Stuff - see assignment

(a) "if" statement

        int i=3;
        if ( i<5 ) {
          System.out.println( " i is less than 5. " );
        }
        else {
          System.out.println( " i is not less than 5. " );
        }

  The computer ignores the indentation.  You shouldn't.

(b) boolean expressions - evaluate to "true" and "false"

     double  jimsHeight = 6.3;
     boolean jimIsSixFeetTall = ( jimsHeight >= 6.0 && jimsHeight <= 7.0 );

  Do examples in class with
    <=
    >=
    ==     tests equality
    !=
    &&     logical "and"
    ||     logical "or"
    !      logical "not"
    ( )    grouping    
     

  WARNING:    DO NOT TYPE  "if (i=2) ... ", but instead use "if (2==i) ..."

  Talk a bit about the algebra of logic operators.

(c) Equality of objects is different than equality of their values.

      Some classes (i.e. String) have methods for testing equality.

         String a = "Hello.";
         String b = "Hello.";
         if ( a == b ) {  
            System.out.println( " 'a' and 'b' are the same. "); }
         else {
            System.out.println( " 'a' and 'b' are different. "); }

     as opposed to 

         if ( a.equals(b) ) {
            System.out.println( " The values of 'a' and 'b' are the same."); }
         else {
            System.out.println( " The values of 'a' and 'b' are different.");}
    
     Note also the string.equalsIgnoreCase() method.

(d) "while" syntax - loops

  int i=1;  
  while ( i<=10 ) {
    System.out.println( " 'i' is " + i );
    i = i+1;
  }

(e) increment, decrement operators.

  i++;
  i--;

(f) temporary variables within {} blocks.   (This is called "scope".)

  ( This is an advanced topic. )

  int i = 3;
  // what is the value of i here ?
  {

     int i=5;
     // what is the value of i here ?
  }

  // what is the value of i here?  (And which 'i' is it?)